The employment of microelectronic circuitry has made possible many advanced developments in missiles and rocketry. Although many improvements have been made, many more improvements are made necessary by the added environmental forces to which a missile or rocket will be subjected. Of particular importance are the techniques used to make secure connections in microelectronic circuitry. The connections must be secured and remain secured and reliable after being secured. The present invention is particularly adapted to making secure connections in microelectronic circuitry, such as from the metallized pads on an integrated circuit (IC), to the metallized pads on the substrate in a hybrid IC. It is important that the described connections be well secured and reliable in any microcircuitry, particularly in hybrid integrated circuits used in the guidance and control of rockets and missiles in flight.
The environmental forces that a missile or rocket is subjected to when it leaves the launcher and those subjected to during in-flight maneuvers means that all the connections in the hybrid integrated circuits in the guidance and control package of the missile or rocket must remain intact. Thus, the system must withstand the hundreds or thousands of gravity forces created as a result of taking off or as a result of fast maneuvers after take off.
The present day art of making the majority of microelectronic circuitry electrical connections from the metallized pads on the IC to the metallized pads on the substrate is effected by wire bonding. In general, aluminum or gold wire of about a mil in diameter is used. The technique of making the wire to adhere or secure to the metallized pads and other conductors on the substrate is by thermocompression bonding or by ultrasonic bonding.
In the thermocompression bonding, the wire is first placed on an area of heated substrate chip. The heat of substrate will help soften the metallized pad to which a connection is to be made. When pressure is put on the wire, from a hard metal tool, the pressure will cause the wire to deform and spread which will assist making contact with the metallized surface of the area. The pressure and the heat causes the two closely contacted metal systems to weld or adhere together. The temperature used during bonding is near the eutectic temperature of the two metals, so a eutectic bond can form. The heating, the pressure, and the mechanical action by the thermocompression of the chip, can cause damage to the microcircuit on the chip. The heat applied during the thermocompression bonding has a time limit and sometime the eutectic bond may not be formed by the two metals.
In ultrasonic bonding, a bonder makes rapid rubbing on the wire against the bonding pad. The rubbing (ultrasonically) causes very high localized temperatures. The heat, hot enough to cause melting and the formation of intermetallics, will cause bonding to be formed. Tool pressure in ultrasonic bonding influences the resonance of the system. The ultrasonic transducer that is used in driving the tool must resonate to produce the bonding energy is difficult to set for maintaining proper resonant frequency. The vibration of the bonder may cause damage to the microcircuit on the chip.
Wire bonding at its best is not as reliable as desired. The wire bonding step is the weakest step in the entire process of IC making. This conclusion has been made because the majority of the field failures of integrated circuits are due to the wire bonding faults.
Advantageous would be a method for producing a flexible, electrical conductor for making connections in microelectronic application, particularly, where the microelectronic circuitry is to be used in a missile or rocket which will be subjected to high gravity environments. An electrical conductor that is flexible and adaptable for making connections in microelectronics circuits with an epoxy adhesive should be of particular interest for high gravity environment use because of the expected lower failure rate as compared to wire bonding techniques.
An object of this invention is to provide an improved electrical conductor for connection in microelectronic application.
Another object of this invention is to provide an electrical conductor of small diameter in the mil range that is adaptable for microcircuitry use.
A further object of this invention is to provide a connecting conductor that can be securely connected in a hybrid integrated circuit with electrical conductive epoxy adhesive.
Still a further object of this invention is to provide a flexible electrical conductor that is more conducive for use in a high gravity environment where rigid connections employing wire bonding techniques have resulted in failures due to wire bonding faults.